Polymeric membranes for gas separation have become an important technology for various industrial refinery processes. In contrast to traditional separation technologies such as cryogenic distillation, pressure swing adsorption and chemical absorption, membrane separations offer several advantages, including lower energy consumption, lower capital investment, and ease of operation (see e.g., Stem S A. Polymers for gas separations: the next decade. J Membr Sci 1994; 94:1-64). Due to a significant growth in interest over the last ˜30 years, numerous polymers have been developed as membranes for a variety of gas separations (see e.g., Baker R W. Future Directions of Membrane Gas Separation Technology. Ind Eng Chem Res 2002; 41:1393-411). An inherent trade-off relationship between permeability and gas selectivity based on empirical observations of available gas transport data has been reported by Robeson (Robeson L M. Correlation of separation factor versus permeability for polymeric membranes. J Membr Sci 1991; 62:165-85; Robeson L M, et al., High performance polymers for membrane separation. Polymer 1994; 35:4970-8) and the theory behind this phenomenon was described by Freeman (Freeman B D. Basis of Permeability/Selectivity Tradeoff Relations in Polymeric Gas Separation Membranes. Macromolecules 1999; 32:375-80). Most of the available gas transport data on polymeric membranes from research laboratories have been measured in the temperature range of 25-35° C. However, for many industrial applications, the ideal operating temperature may vary significantly from ambient conditions. For example, a high operation temperature (150-300° C.) is required to improve the thermal efficiency for H2 separation from pre-combustion syngas in the Integrated Gasification Combustion Cycle (IGCC) system for electricity production (Merkel T C, et al., Carbon dioxide capture with membranes at an IGCC power plant. J Membr Sci 2012; 389:441-50; O'Brien K C, et al., Towards a pilot-scale membrane system for pre-combustion CO2 separation. Energy Procedia 2009; 1:287-94). These harsh conditions eliminate most polymer membranes from consideration due to thermal instabilities that lead to degradation and loss of mechanical properties (Robeson L M, et al., Synthesis and Dynamic Mechanical Characteristics of Poly(Aryl Ethers). Appl Polym Symp 1975; 26:375-85).
Polybenzimidazoles (PBIs), initially developed by Marvel, are well known for their outstanding thermal stability, often exhibiting glass transition temperatures greater than 400° C. as well as flame retardance and chemical stability (Ueda M, et al., Poly(benzimidazole) synthesis by direct reaction of diacids and diamines. Macromolecules 1985; 18:2723-6; Vogel H, et al., Polybenzimidazoles, new thermally stable polymers. J Polym Sci 1961; 50:511-39). Due to these characteristics, they are promising candidates for gas separation membranes that can be used at high temperatures. Membranes prepared from a commercial polybenzimidazole, CELAZOLE™, have been shown to have attractive gas transport properties (Berchtold K A, et al., Polybenzimidazole composite membranes for high temperature synthesis gas separations. J Membr Sci 2012; 415-416:265-70; Pesiri D R, et al., Thermal optimization of polybenzimidazole meniscus membranes for the separation of hydrogen, methane, and carbon dioxide. J Membr Sci 2003; 218:11-8). CELAZOLE™ (sometimes referred to as m-PBI in the literature) (Id.; O'Brien et al., Energy Procedia 2009; 1:287-94) is prepared from 3,3′-diaminobenzidine and isophthalic acid. However, polybenzimidazoles based on the 3,3′-diaminobenzidine monomer have very limited solubilities in common solvents due to their rigid rod structures and intermolecular hydrogen bonding (Li X, et al., Synthesis and Characterization of a New Fluorine-Containing Polybenzimidazole (PBI) for Proton-Conducting Membranes in Fuel Cells. Fuel Cells 2013; 13:832-42). For instance, m-PBI is only partially soluble in dimethylacetamide and insoluble in other common solvents, and PBIs based on 3,3′-diaminobenzidine and terephthalic acid are insoluble in common organic solvents (Vogel et al., J Polym Sci 1961; 50:511-39). Structural modification of polymer backbones to include flexible linkages usually increase the solubility of PBIs (Kumbharkar S C, et al., High performance polybenzimidazole based asymmetric hollow fibre membranes for H2/CO2 separation. J Membr Sci 2011; 375:231-40). However, a reduction in rigidity causes a decrease in the glass transition temperature, thus compromising the high temperature properties of these glassy polymers (Kumbharkar S C, et al., Enhancement of gas permeation properties of polybenzimidazoles by systematic structure architecture. J Membr Sci 2006; 286:161-9). What are thus needed are new synthetic methods towards PBIs as well as new PBI derivatives. The compositions and methods disclosed herein address these and other needs.